1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with a cross brace incorporated in the water cross of a fuel assembly so as to eliminate most of the deflection of the interconnected water cross panels and outer flow channel of the fuel assembly due primarily to the internal pressure of the outer flow channel relative to the reactor system pressure.
2. Description of the Prior Art
Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work.
A plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a neutron moderator. The fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. The control rods is insertable within the fuel assemblies for controlling the reactivity of the core. Each such cluster of four fuel assemblies surrounding a control rod is commonly referred to as a fuel cell of the reactor core.
A typical fuel assembly in the cluster is ordinarily formed by a N by N array of the elongated fuel rods supported between upper and lower tie plates in laterally spaced-apart relation. In a fuel assembly of this type the fuel rods in the central region of the assembly may be undermoderated and overenriched. In order to remedy this condition by increasing the flow of moderator water through this region of the assembly, an elongated, centrally-disposed stiffening device with vertical water passageways, such as illustrated and described in Canadian Pat. No. 1,150,423 and U.S. Pat. No. 4,478,786 to Ingmar Andersson et al, or an elongated, centrally-disposed water cross, such as illustrated and described in the U.S. patent applications cross-referenced above, can be used. The water cross being in the form of a cruciform water flow channel provides a centrally-disposed cross-shaped path for the flow of subcooled neutron moderator water within the channel along the lengths of adjacent fuel rods.
In the central water cross of the above cross-referenced Barry et al patent application, numerous sealed passages were formed through its radial panels to permit coolant flow between the separate sections or subgroups of fuel rods of the assembly to provide hydraulic pressure equalization therebetween. To overcome certain difficulties in manufacturing panels with the passages and ensuring against weld failures therein, the above cross-referenced Doshi patent application provided a water cross which eliminates the passages through the radial panels, substituting in their place imperforate dimples which spaced-apart and interconnected the angles which made up the panels. Instead of the passages in the panels, Doshi formed coolant flow paths through each of a series of elongated, solid, vertically-extending ribs. Each rib is located between and interconnects the respective inner wall of the annular outer flow channel and the outer vertical edge of each water cross panel.
Notwithstanding the improvements fostered by the cruciform water cross flow channel of the Doshi patent application, it overlooked another problem which, if left uncorrected, limits the useful lifetime of the outer flow channel. During normal reactor operation, pressures inside the water cross channel and among the four subgroups of fuel rods within the outer channel are significantly greater than the pressure in the reactor system exterior thereto. The higher pressure in the outer channel especially, relative to the reactor pressure, tends to force the outer channel to take on a more cylindrical shape which, in turn, pulls radially outward on the radial panels of the water cross and tends to unbend the angles of the panels.
Consequently, the need exists for some means of counteracting such creep and distortion of the outer channel and water cross of the fuel assembly so as to prolong the useful life of the outer channel.